The present invention relates to a method for producing steel bars or wire rods from steel materials having a square or round cross section with a high productivity and using a smaller consumption of energy with respect to heating and power required for plastic deformation than is required in the carrying out of known methods for producing similar finished product. The process disclosed and claimed herein also satisfies gauge and quality requirements.
According to a known procedure for producing steel bars or wire rods, molten steel is continuously cast into blooms, which are subjected to break-down rolling into billets, and the billets are sent to a bar mill or wire rod mill here the billets are followed by 10 to 30 continuous hot rolling mill stands into bars or wire rods of desired diameters or gage.
According to another known procedure for producing steel bars or wire rods, blooms are sent directly to the bar mill or wire rod mill where the blooms are rolled by 10 to 30 continuous hot rolling mill stands into the desired gages. Some detailed descriptions of the known art will now be made.
FIG. 1 illustrates a conventional bar or wire rod rolling process using a four-strand mill, the steel extracted from the heating furnace 1 is transferred to the rough rolling mill train 2. The material is elongated by alternate reducing forces exerted in both the vertical and horizontal directions by roll grooves such as, for example, of diamond cross section, and square cross section. When a diamond groove is used first, it is necessary to turn the material 90.degree. at the inlet of the square groove. For this purpose twist guides are provided at predetermined positions to twist the material being rolled.
Thus, according to the conventional procedure, a multi-strand rolling using 2 to 4 rough rolling mill trains composed of a plurality of horizontal roll mill stands is performed using the twisting operation described above.
Similar twisting/rolling is also done in the intermediate rolling mill train 3, and only in the finish roll mill train the single strand rolling (without twisting) is performed using a "block-mill" 4. A block-mill is a rolling mill train in which horizontal rolls and vertical rolls are alternately arranged, or a rolling mill train in which the roll axis is inclined .+-.45.degree. with respect to the vertical axis, then the material is subjected to controlled cooling in the controlled cooling section 5, 6 and finally coiled on the coiler 7.
In the multi-strand rolling system as described above, it is essential to completely interrelate the delivery speeds of the material between preceding stands and subsequent stands all though the rough rolling, intermediate rolling and finishing mill stands. Then, it is essential that the amount of material delivered by one train of the No. 1 stand of the rough rolling mill train be in accordance with that delivered from one train of the final stand of the finishing rolling train. Otherwise the material so processed would suffer defects such as, for example, burrs and breakings between the stands, thus preventing the rolling operation entirely.
Therefore, the cross sectional dimension of the material at the inlet of the rough rolling mill train is inevitably determined by that of the final product at the finishing stand and the rolling speed at the outlet thereof. For example, when a steel wire of 5.5 mm diameter is to be produced using an ordinary block mill having a maximum finishing speed of 60 M/S, the cross sectional dimension of the starting material is limited to 120 mm square maximum from the aspect of the roll life and the lowest material temperature during the rolling process (Ar.sub.3 point) to be assured.
Thus, according to the conventional process described above, in which the steel material is reduced in its cross section by a rolling mill train equipped with 10 to 30 or more mill stands, the elongation of the material between the starting material and the final product is normally a factor of 500 to 600 times. Therefore, the ratio of the rolling speed at the initial rolling stand to that at the final rolling stand is 1 to 500-600.
In popular wire rod rolling mills, the rolling speed at the final rolling mill cannot be increased beyond about 60 m/sec. and the rolling speed at the initial rolling stand in proportion to this rolling speed is surprisingly as low as 0.1 m/sec.
Therefore, the temperature of the material at the initial portion of the rolling mill train drops rapidly to a temperature so low that the plastic deformation is no longer possible. For compensating this temperature lowering, the steel material must be heated to temperatures high enough to compensate for this expected temperature drop.
However, the steel material cannot be heated to a temperature beyond the melting point, so that compensation for the expected temperature drop, which would require heating of the material to a temperature beyond its melting point, is practically impossible.
For all these reasons, the conventional procedure has an inherent limitation with respect to the elongation rate, as compared between the starting material and the final product, which is applicable in the rolling mill train. Even in the existing highest-level wire rod rolling mill train, the largest applicable cross section of the starting material is 120 mm to 150 mm square.
Therefore, in the conventional process, a starting material of small cross sectional dimension is used so as to decrease the difference in the rolling speed between the initial portion and the finishing portion of the rolling mill train, thereby alleviating the necessity of lowering the rolling speed required by the lowering of the temperature of the steel material at the initial portion of the rolling mill train.
However, there is a problem with this approach of using a small cross sectional diameter starting material. The blooms and billets which are starting materials for production of bars and wire rods have, in the past, been obtained by breaking down ingots, but this conventional art has been increasingly replaced by the continuous casting of molten steel directly into blooms and billets.
For example, if the bloom is prepared by the continuous casting process, there is a requirement that the cross sectional dimension of the bloom thus obtained should be as large as possible in order to maximize productivity. Also in cases where high-quality wire rods are to be produced from continuously cast blooms, such high-quality blooms can be obtained only when blooms of large cross sectional dimension are continuously cast.
Further, when the blooms prepared by continuous casting have surface defects, these surface defects must be removed by grinding. If the surface area to be removed is determined by a predetermined proportion to the total bloom surface, the surface area to be removed per unit weight of the bloom becomes smaller as the cross sectional dimension of the continuously cast blooms increases. This is because the surface area per unit weight of the material increases as the cross sectional dimension of the bloom decreases.
As explained above, when blooms prepared by continuous casting of molten steel are used as the starting material for the production of bars or wire rods, the desired productivity of the continuous casting process or the desired efficiency of the surface defect removal cannot be achieved without increasing the cross sectional dimension of the starting material.
The advantage of blooms of large cross sectional dimension for obtaining a high productivity and an efficient removal of the surface defects can be achieved also in the process for obtaining blooms by breaking down steel ingots.